1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to an optical transmission system that transports optical signals with optical wavelength-division multiplexing (WDM) techniques.
2. Description of the Related Art
WDM technology enables many transmission signals to be carried over a single fiber-optic medium, assigning different optical wavelengths to different communication channels. Recent years have seen a remarkable progress in development of WDM networks as a key technology for data communication infrastructures. A widely used WDM backbone structure is ring network systems, which is formed from a plurality of WDM nodes interconnected in a ring topology. In such a WDM ring system, each node functions as an optical add/drop multiplexer (OADM), which drops (removes) and adds (inserts) optical signals with particular wavelengths from/to the multi-channel backbone traffic. Nodes are linked by two or more parallel fiber cables so that one signal path can be replaced with another path in case of failure in the WDM ring network.
FIG. 11 shows how OADMs operate in a WDM system. The illustrated WDM ring network 100 involves four nodes 101 to 104 interconnected by optical transmission lines L1 and L2 in a dual ring topology. Wavelengths λ1 to λ4 are added at the nodes 101 to 104, respectively. The WDM signal on the ring contains those wavelength components λ1 to λ4, and the nodes 101 to 104 can drop any desired wavelength component out of the WDM signal circulating along the ring. In the present example, a wavelength λ1 is dropped at one node 102, while wavelengths λ1 and λ2 are dropped at another node 103. Notice that the same wavelength λ1 is dropped at two different nodes 102 and 103. This type of ring structure is called multidrop configuration.
FIG. 12 shows failover in the WDM ring network 100. The first line L1 running in the counterclockwise direction is used as a working path, and the second line L2 running in the clockwise direction is used as a protection path. Both the working path and protection path convey all four wavelengths λ1 to λ4 to deliver the same service data. This signal is referred to as WDM signal D1.
Suppose now that the working line Lla has encountered a link failure at a point between nodes 101 and 102 during normal operation. To recover from the failure, the system switches the working path from the current line L1 to the other line L2, the latter having been assigned as a protection path. The new working path circumvents the failure of line L1, thus enabling the system to continue its service.
The WDM ring network 100 has multidrop capabilities and takes a non-revertive “1+1” switching architecture. Since this protection system always assigns two paths to carry the same service signals in preparation for a possible link failure, the actual usage of bandwidth is at most 50 percent even in normal operation, failing to use the full available network capacity efficiently.
Some researchers propose a transmission device for a WDM dual-ring network with protection switching functions. See, for example, Japanese Patent Application Publication No. 2003-244072, paragraphs 0020 to 0025, FIG. 1. The proposed device has two output interfaces disposed between add/drop multiplexer modules with a switch circuit. When a path failure occurs, the decision of which output interface to use is made on the basis of destinations of user data and the information about that failure. This device, however, lacks consideration for the bandwidth usage in normal situations.